Why fruit is sweet, veg is starch?

Both plants use polysaccharides as storage substances. In photosynthesis, plants build up trioses sugars and then bind the sugars together by condensation (production of water). The fruit is actually a 'seed' of the plant and it contains sucroses mainly (monosaccharides), so that they can easily be digested and produce energy for the growth of the new seed into a plant. Young plants cannot take the time in breaking polysaccharides to monosaccharides.